In the production of semiconductor circuits and especially integrated circuits, it is a common practice to transfer a pattern to a semiconductive substrate by an optical system which focuses a greatly reduced image of a mask upon this substrate. The substrate can be made photosensitive by an appropriate coating and can be activated through the imaging process.
In general, the apparatus utilized for this purpose includes a mask having opaque and transparent regions in accordance with the pattern, a light source for transluminating the mask, an objective lens system for focusing the transluminating beam upon a portion of the semiconductive substrate, and if desired, means for repeatedly imaging the mask for coupling the pattern thereon on successive portions of the substrate, each portion of which is adapted to form a respective integrated circuit upon coating of the substrate to isolate these portions.
Alternatively, the patterns may be reproduced on a substrate to form a large-scale integrated circuit.
Essential to the process is the sharp focusing of the image of the mask upon this substrate. To this end, means may be provided, utilizing the projection objective, for focusing a portion of the image on the substrate, onto a detector plane which allows the sharpness of the projected image to be determined. This latter projection can be termed a back projection through the objective lens system.
In the apparatus described in German patent document -- Open Application No. DE-OS 29 05 635, the back-projected image is an image of two parallel edges defining a window and associated in turn with alignment marks of the mask.
The means for back-projecting the image includes a semitransparent mirror disposed between the mask and the projection objective which trains the back-projected image on an evaluation means sensing the back-projected image in a detection plane which is normal to the edges of the window.
The evaluating circuit produces electrical signals proportional to the intensity of the image, and the slope of the electrical signals corresponding to the window edges is evaluated with the resulting value being used to control the relative positions of the mask and the workpieces or substrate in the direction of the optical axis.
The image upon the substrate, whose sharpness is determined by the focusing conditions and which is to be determined by the means just described, need not, however, include or consist of an image of the alignment marks of the mask and thus it is possible to provide the mark upon the substrate itself and to evaluate the sharpness and focus in terms of the gradient of the sensed patterns so that the resulting electrical signals will be a measure of the focusing state of the device.
With the conventional apparatus, a deviation in the position of the substrate from its ideal position in the direction of the optical axis may be detected so that the substrate is shifted in position until the slope of the intensity measurement of the image of the pattern reaches a maximum value. The disadvantage of this system is that a substrate generally must be shifted beyond this optimum value before the optimum value is detected and thus must be returned in the opposite direction. In other words, the actual positioning of the substrate requires a hunting adjustment; typically during adjustment a number of shifts of the substrate in the direction of the optical axis may be required in a time-consuming process.